In the prior art, DE-PS 3 334 470 shows a single-line inking unit for a rotary offset printing machine comprising a plurality of ink transport and inking rolls. The damping medium is fed to the first inking roll or the last inking roll or both. The ink is fed to a spreader roll associated with a first and second inking roll, five inking rolls being arranged successively at the plate cylinder in each case with a rider roll being disposed between each of them. The first rider roll and the third rider roll are constructed as distributor rolls. With the described arrangement of the five inking rolls and rider rolls, the pliability of the offset printing ink on the inking rolls decreases or is at least maintained from one inking roll to the next as viewed in the direction of rotation of the plate cylinder. The distribution behavior in the inking unit is thus optimized in that mottling can be extensively avoided.
If ink is supplied by only one flow of ink to the inking rolls, then if there are any variations in the proportion of damping medium at the distribution points situated in the flow of ink, the ink distribution ratios change. If the proportion of damping medium changes at the distribution points of the flow of ink due to a change in the supply of damping medium by a damping unit, particularly in the case of damping direct to the inking unit, then the ink supply (ink feed) must be readjusted to give the same application of ink to the printing plate. If the proportion of damping medium changes in the ink flow distribution points due to different quantities of damping medium on the printing plate, as a result of the distribution of printing and non-printing area components, then pulsation occurs in the flow of ink and produces mottling.
Inking units are generally known which divide up the flow of ink so that one component of the flow of ink is fed to the first inking roll or rolls and a second component is fed to the last inking rolls (see, for example, DE-PS 2 302 261, DE-PS 3 434 647, DE-PS 3 143 909). These inking units comprise a relatively large number of ink transport rolls in conjunction with one distributor roll, from which the two flows of ink are transferred to three or more inking rolls arranged successively at the plate cylinder. The distributor roll is coupled on the one hand to the ink feed system and on the other hand to branch rolls which feed the ink sub-flows to the inking rolls. In these inking units, readjustment of the ink feed in the event of variation of the ink distribution conditions in one component of the ink flow is not necessary but there is no guarantee of the ink transfer proportions of the individual inking rolls being graduated. In the extreme case, the ink transfer components of the last inking rolls become greater than those of the first inking roll or rolls, and this leads to the known mottling phenomena.